The invention relates to a method and a valve apparatus for counteracting harmful pressure pulses caused by rapid retardation of a liquid in a hydraulic system comprising a pump for the supply of liquid to a conduit stretch containing a non-liquid-filled volume.
Pressure pulses may arise in all hydraulic systems wherein a liquid is subjected to a rapid velocity change. If adequate measures are not taken, one may risk that the pressure pulses burst pipes, pumps, etc. An example of such systems is fire water systems on offshore installations. Such systems comprise high-power pumps supplying large quantities of water via ring lines at several levels to the automatic extinguishing plant forming part of the system. In these systems one may risk that the highest-lying part of the pipe system is partly gas-filled (air, water vapour etc.) at the moment when the high-power pumps start up. This is primarily due to the fact that the automatic extinguishing plant has to start up as quickly as possible in order to minimise the effect of a possible fire attempt, whereas it takes time to start up the pumps. The driving pressure of the pumps may typically be of the order of 10-15 bar, whereas the pressure in the gas-filled part of the pipe system may be substantially less than I atmosphere as the pumps start up. The gas volume therefore must be reduced to a fraction before the pneumatic counterpressure is so high that the liquid begins to retard. Consequently, the liquid will have to be slowed down over a very short stretch, and the pressure pulse will become correspondingly strong.
Traditionally, essentially four different methods are used for reducing the risk for pressure impacts in such fire water systems. One method is to use a vacuum switch which automatically admits air at atmospheric pressure into the relevant pipe as soon as water is tapped from the pipe system. The uncertainty of this method is that one is dependent on the pumps not starting before the quantity of air which has been admitted to the system, can secure a satisfactory smooth retardation of the water.
Another method is to provide a liquid-filled buffer tank in the system. When water is tapped from the system before the pumps have started, the buffer tank will supply sufficient liquid to the system to secure that a too low pressure in parts of the system does not arise.
A third method is to have an air-filled buffer tank in the system. When the pumps start up, the water will gradually fill the buffer tank, and the air in the buffer tank will be compressed and gradually cause the liquid velocity to be reduced. By choosing a suitable size of the buffer tank, one will be able to achieve that the liquid flow is slowed quietly down.
A fourth method which is used, is to place a low-lying buffer tank in the system, wherein the tank is pressurised with compressed air.
The drawbacks of these known methods are either that they do not give a sufficient security, or that they are expensive and space-demanding, or that it takes an unduly long time to achieve normal operating conditions in the fire water system.
The object of the invention is to provide a simple and cost-efficient method for counteracting harmful pressure pulses in different types of hydraulic pipe systems wherein there is a risk that the pipe system may be partly gas-filled when pumps or corresponding devices are put into function.
For achieving the above-mentioned object there is provided a method of the introductorily stated type which, according to the invention, is characterised by the steps of
sensing the pressure in said volume in the conduit,
supplying compressed gas to the volume when the pressure sinks below a given first level, and seeing that the pressure is maintained at a desired value which is at least equal to the first level, and
shutting-off the supply of compressed gas to the volume when the pressure therein has risen to a given second level.
In an advantageous embodiment of the method an increased quantity of compressed gas is supplied to said volume when the pressure therein, as a consequence of an external influence, rises after having fallen to the first level.
According to the invention there is also provided a valve apparatus of the introductorily stated type which is characterised in that it comprises a main valve having an inlet connected to a reservoir for pressurised gas, a first valve chamber connected in use to said volume, and a regulating unit for controlling the supply of pressurised gas to the first chamber, which unit comprises
a first device arranged to open for the supply of pressurised gas via the main valve to the first chamber when the pressure therein sinks under an activating value, and thereafter to maintain the pressure in the chamber at a desired value,
a second device causing increased supply of pressurised gas to the first chamber when the pressure therein rises after having fallen to the activating pressure, and
a third device causing closing of the main valve and therewith shutting-off of the gas flow to the first chamber when the pressure therein has risen to a closing value.
Thus, in the present invention there is used an active pneumatic system to minimise pressure impacts or surges in hydraulic systems. The method offers substantial advantages in relation to the prior art in that it is simple and inexpensive and sees that normal operating conditions are established quickly. The valve apparatus according to the invention is a regulating valve which senses when the liquid (water) is put in motion, and provides for quickly establishing a pneumatic counterpressure at the downstream side of the liquid. The movement of the liquid will cause the pneumatic pressure to increase beyond the pressure of the supplied pressurised gas, and gradually cause the liquid flow to retard. The purpose of actively supplying compressed gas at the downstream side of the liquid is to minimise the pressure pulse by seeing that the distance travelled by the liquid flow during the retardation phase, is sufficient to counteract harmful pressure pulses in the system.
In the case of a fire water system of the above-mentioned type it may, in principle, be sufficient to install only one valve apparatus at the highest point of the pipe system. This apparatus preferably should be arranged to maintain a given minimum overpressure in the pipe system. If water is tapped from the system before the pumps have been started, that part of the pipe system which is situated between the valve apparatus and a first subjacent ring line, will be able to function as a buffer volume which is gradually filled with gas. Care must be taken that this buffer volume is sufficiently large that it may not be able to be emptied completely of water before the pumps start up. By adapting the overpressure to the construction of the piping, one can avoid that gas pockets arise in other parts of the pipe system. The additional function of the valve apparatus will be to secure that the total gas quantity which is supplied to said buffer volume, is sufficient that the liquid gets such a smooth retardation that harmful pressure pulses are avoided. This is secured in that the valve apparatus observes that the pumps start up, and provides for an instantaneous supply of gas into the buffer volume, for thereafter to break the supply of gas as soon as the buffer volume has been supplied with a metered quantity of gas or the pressure of the buffer volume has reached a given level.